1. Field of the Invention
The present invention relates to a socket for electrical parts for detachably accommodating and holding an electrical part such as a semiconductor device (called as “IC package” hereinlater), to conduct an electric performance test and examination of such electrical part.
2. Related Art of the Invention
As a conventional “socket for electrical part,” there have been provided an IC socket for detachably holding an IC package as “electrical part.”
The IC package includes, for example, one having a plurality of terminals on a lower surface of strip like package body.
On a socket body of the IC socket, a plurality of probe pins are provided. The probe pin is designed to establish an electrical connection between a printed circuit board and the IC package terminal.
As a conventional probe pin, there is, for example, one which comprises a vertically movable upper side contact member and a lower side contact member disposed at an upper and lower inner sides of a tubular member and a spring disposed between both contact members. Both contact members are urged by the spring to push away from each other.
The probe pin is designed to be accommodated in the socket body. FIG. 18 shows an example of the accommodating structure. That is, the structure has 2 plates, plates 1 and 2. In the plates 1 and 2, holes 1a and 2a with stepped portion are formed, respectively. When the plates 1 and 2 are superimposed, the probe pin can be accommodated in a pair of step holes 1a and 2a. 
After the probe pin is accommodated in the pair of step holes 1a and 2a, the probe pin is prevented from coming off upward or downward by the existence of the stepped portions 1b and 2b. 
The plates 1 and 2 are usually produced by molding of resin using a mold die. In these days, however, increasing number of IC packages are made by small-lot production. Therefore preparing a mold die which can only be used for producing a socket for electrical part to be made by the small-lot production may cause increase in production cost, being uneconomical.
In order to bring down the cost of the socket for electrical part to be produced by the small-lot production, some socket parts made of the same resin as used in the same molded parts is considered to be produced through machine work, for example, a cutting process etc. instead of molding processes using mold dies.
But, when a number of stepped holes 1a, 2a is made, through cutting work, in the above mentioned plates 1 and 2, it is difficult to make a depth (or height) D constant of the larger diameter hole of the stepped holes 1a and 2a. Therefore, there occurs a problem of dimensional difference (dispersion) in the stepped portion 1b, 2b of the stepped holes 1a, 2a. 
In a case where the probe pin is installed in such conventional plates 1, 2 and used as a socket, there is a fear that performance test of the IC package can not be carried out stably and certainly because contact pressure of the probe pin tends to fluctuate and can not be controlled within a prescribed range.
The dimensional difference arisen from the cutting process of the stepped holes 1a, 2a of the plates 1, 2 grows larger as the number of terminals increases.
In addition, in the conventional socket for electrical parts, the plates 1, 2 are made of a synthetic resin having excellent insulation performance and physical strength, so that the thermal expansion coefficient of the synthetic resin is larger than that of the insulating material (for example, epoxy resin with glass fiber substrate, etc.) of the printed circuit board. The amount of deformation of the plates 1, 2 is different from that of the circuit board at a high temperature during burn in testing. There is a fear that the pitch of the stepped holes 1a, 2a of the plates 1, 2 into which probe pin is accommodated is different from that of an electrode on the circuit board against which the probe pin abuts. If the burn-in testing is conducted under the condition that the probe pin is slightly misaligned from a prescribed position of the electrode of the circuit board, electrical connection between the probe pin and the electrode of the printed circuit board may not be established at worst. Concerns mentioned above grow larger in these days, as the number of terminals of the IC package is increasing and the pitch of the terminals becomes narrower.